Wednesday, November 27, 2013

Re: [Geology2] Ancient Minerals: Which Gave Rise to Life?



that is probably the best description of the olivine series that I have ever seen that did not go into technical details and involve phase diagrams. 

john


On Wed, Nov 27, 2013 at 3:42 AM, MEM <mstreman53@yahoo.com> wrote:
 

Actually "olivine"(Mg,Fe2SiO4) is a grandfathered "mineral" by the IMA because it was known to the ancients long before it was possible to distinguish it chemically. Olivine was named for it's unripened, olive-like color. Most ancient olivine was from mg-rich magmas so it was usually very green and given the gem name peridot when so graded. ( Trivia: dunnite is the massive rock form of olivine and given the name owing to its similar color to--Dung.)  

Olivine is technically a "solid solution series" of any porportion of the two minerals: Magnesium Silicate and Iron Silicate--Forsterite (Mg2SiO4) and Fayallite (Fe2SiO4).( I'd have to check to be sure but, I think mineralogy standards allow up to 10% of the other end member and still be classified "pure".   I also believe fayallite is much more rare than forsterite owing to concentrations of mg in the upper mantle/lower crust but the opposite in the asteroid belt/meteorites.

Both molecules have a similar melting point and a similar atomic diameter so they freely substitute in the crystal matrix very well.  The more magnesium, the greener the olivine*.  Peridot is technically only the mg-rich olivine. The more iron in the matrix, the more yellow-brown the olivine color is. Most olivine from meteorites is iron rich and is yellow brown. A notible exception is the pallasite meteorite "Esquel" which has the greenest gem-quality peridot.

Olivine as mix of the two minerals is fairly stable at depths but at the surface it weathers to serpentine in the presence of hot water.  The original quoted article is in error in that the green color is due to magnesium-- not iron.  It may be discussing how the little bit of iron there was in near surface olivine was released in weathering.  Science writers often make these kind of mistakes.

Eman

*in the carbonates, calcium carbonate (calcite) (being white) forms a solution series with magnesium carbonate ( (rhodochrosite) (being deep rose-pink). Dolomite (Ca,Mg2 CO3) (being pink) is in the center of the series.  Furthermore, the mineral siderite is the iron carbonate but rarely mixes with calcium or magnesium carbonate although all of carbonates are water soluble.  Siderite is yellow to brown. 

The point is that color is a derivative of the metal cation's degree that it is "complexed by ligands and the family of ligands involved".  The "color of molecules"  was one of my chemistry research projects 43 years ago and wow I finally got to share some of it!!  I hope it was worth the wait...te he he.

Eman


On Tuesday, November 26, 2013 2:42 PM, Kim Noyes <kimnoyes@gmail.com> wrote:
 
So forsterite is olivine because olivine is peridot and this article says peridot is also forsterite?


On Tue, Nov 26, 2013 at 6:16 AM, Lin Kerns <linkerns@gmail.com> wrote:
 
alt

The magnesium silicate forsterite was one of the most abundant minerals in the Hadean Eon, and it played a major role in Earth's near-surface processes. The green color of this mineral (which is also known as the semi-precious gemstone peridot, the birthstone of August) is caused by small amounts iron. The iron can react with seawater to promote chemical reactions that may have played a role in life's origins. (Credit: Robert Downs, University of Arizona, Ruff Project)

Ancient Minerals: Which Gave Rise to Life?

Nov. 25, 2013 — Life originated as a result of natural processes that exploited early Earth's raw materials. Scientific models of life's origins almost always look to minerals for such essential tasks as the synthesis of life's molecular building blocks or the supply of metabolic energy. But this assumes that the mineral species found on Earth today are much the same as they were during Earth's first 550 million years -- the Hadean Eon -- when life emerged. A new analysis of Hadean mineralogy challenges that assumption.
The work is published in American Journal of Science.
Carnegie's Robert Hazen compiled a list of every plausible mineral species on the Hadean Earth and concludes that no more than 420 different minerals -- about 8 percent of the nearly 5,000 species found on Earth today -- would have been present at or near Earth's surface.
"This is a consequence of the limited ways that minerals might have formed prior to 4 billion years ago," Hazen explained. "Most of the 420 minerals of the Hadean Eon formed from magma -- molten rock that slowly crystallized at or near Earth's surface -- as well as the alteration of those minerals when exposed to hot water."
By contrast, thousands of mineral species known today are the direct result of growth by living organisms, such as shells and bones, as well as life's chemical byproducts, such as oxygen from photosynthesis. In addition, hundreds of other minerals that incorporate relatively rare elements such as lithium, beryllium, and molybdenum appear to have taken a billion years or more to first appear because it is difficult to concentrate these elements sufficiently to form new minerals. So those slow-forming minerals are also excluded from the time of life's origins.
"Fortunately for most origin-of-life models, the most commonly invoked minerals were present on early Earth," Hazen said.
For example, clay minerals -- sometimes theorized by chemists to trigger interesting reactions -- were certainly available. Sulfide minerals, including reactive iron and nickel varieties, were also widely available to catalyze organic reactions. However, borate and molybdate minerals, which are relatively rare even today, are unlikely to have occurred on the Hadean Earth and call into question origin models that rely on those mineral groups.
Several questions remain unanswered and offer opportunities for further study of the paleomineralogy of the Hadean Eon. For example, the Hadean Eon differs from today in the frequent large impacts of asteroids and comets -- thousands of collisions by objects with diameters from a mile up to 100 miles. Such impacts would have caused massive disruption of Earth's crust, with extensive fracture zones that were filled with hot circulating water. Such hydrothermal areas could have created complex zones with many exotic minerals.
This study also raises the question of how other planets and moons evolved mineralogically. Hazen suggests that Mars today may have progressed only as far as Earth's Hadean Eon. As such, Mars may be limited to a similar suite of no more than about 400 different mineral species. Thanks to the Curiosity rover, we may soon know if that's the case.

Story Source:
The above story is based on materials provided by Carnegie Institution.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
  1. R. M. Hazen. Paleomineralogy of the Hadean Eon: A preliminary species list. American Journal of Science, 2013; 313 (9): 807 DOI: 10.2475/09.2013.01

Carnegie Institution (2013, November 25). Ancient minerals: Which gave rise to life?. ScienceDaily. Retrieved November 26, 2013, from http://www.sciencedaily.com­ /releases/2013/11/131125164814.htm





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